Ultrafast vibrational spectroscopy (2D-IR) of CO₂ in ionic liquids: Carbon capture from carbon dioxide’s point of view

The CO₂ν₃ asymmetric stretching mode is established as a vibrational chromophore for ultrafast two-dimensional infrared (2D-IR) spectroscopic studies of local structure and dynamics in ionic liquids, which are of interest for carbon capture applications. CO₂ is dissolved in a series of 1-butyl-3-methylimidazolium-based ionic liquids ([C₄C₁im][X], where [X]⁻ is the anion from the series hexafluorophosphate (PF₆⁻), tetrafluoroborate (BF₄⁻), bis-(trifluoromethyl)sulfonylimide (Tf₂N⁻), triflate (TfO⁻), trifluoroacetate (TFA⁻), dicyanamide (DCA⁻), and thiocyanate (SCN⁻)). In the ionic liquids studied, the ν₃ center frequency is sensitive to the local solvation environment and reports on the timescales for local structural relaxation. Density functional theory calculations predict charge transfer from the anion to the CO₂ and from CO₂ to the cation. The charge transfer drives geometrical distortion of CO₂, which in turn changes the ν₃ frequency. The observed structural relaxation timescales vary by up to an order of magnitude between ionic liquids. Shoulders in the 2D-IR spectra arise from anharmonic coupling of the ν₂ and ν₃ normal modes of CO₂. Thermal fluctuations in the ν₂ population stochastically modulate the ν₃ frequency and generate dynamic cross-peaks. These timescales are attributed to the breakup of ion cages that create a well-defined local environment for CO₂. The results suggest that the picosecond dynamics of CO₂ are gated by local diffusion of anions and cations